Exploring the Impact of the HOMO–LUMO Gap on Molecular Thermoelectric Properties: A Comparative Study of Conjugated Aromatic, Quinoidal, and Donor–Acceptor Core Systems

Thermoelectric materials have garnered significant interest for their potential to efficiently convert waste heat into electrical energy at room temperature without moving parts or harmful emissions. This study investigated the impact of the HOMO–LUMO (H-L) gap on the thermoelectric properties of three distinct classes of organic compounds: conjugated aromatics (isoindigos (IIGs)), quinoidal molecules (benzodipyrrolidones (BDPs)), and donor–acceptor systems (bis(pyrrol-2-yl)squaraines (BPSs)). These compounds were chosen for their structural simplicity and linear π-conjugated conductance paths, which promote high electrical conductance and minimize complications from quantum interference. Single-molecule thermoelectric measurements revealed that despite their low H-L gaps, the Seebeck coefficients of these compounds remain low. The alignment of the frontier orbitals relative to the Fermi energy was found to play a crucial role in determining the Seebeck coefficients, as exemplified by the BDP compounds. Theoretical calculations support these findings and suggest that anchor group selection could further enhance the thermoelectric behavior of these types of molecules.